Progress has been made toward the development of a trivalent live attenuated virus vaccine consisting of (1) a respiratory syncytial virus (RSV) Subgroup A component; (2) an RSV subgroup B component; and (3) a parainfluenza virus type 3 (PIV3) vaccine component. After evaluation of five live attenuated subgroup A virus vaccine candidates, the RSV A2 Subgroup A candidate cpts248/404 has been found to be satisfactorily attenuated, phenotypically stable, poorly transmissible, and immunogenic in seronegative infants and children over the age of six-month. As a consequence of this favorable constellation of properties, we initiated studies in the target population of one month old infants for the first time in our live attenuated RSV vaccine program. The RSV subgroup B candidate vaccine, RSV B1 cp52, and two of its more attenuated derivatives were found to be over-attenuated for fully susceptible infants and children and for this reason will not be pursued further. Contributing to the over-attenuation of the cp52 virus was a large spontaneous deletion mutation that ablated the synthesis of two virion surface proteins. A live attenuated PIV3 candidate vaccine, JS cp45, produced by NIH was satisfactorily attenuated, immunogenic, and phenotypically stable in seronegative infants and children, and studies with simian Vero cell-grown vaccine produced by our CRADA partner, Wyeth-Lederle-Praxis, have been initiated this past year. Once fully acceptable monovalent vaccines are identified, studies with bivalent or trivalent vaccines will be initiated.